Influence of Photoperiod and Leaf Age on Crassulacean Acid Metabolism in Portulacaria afra (L.) Jacq.

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Abstract
The possibility that crassulacean acid metabolism (CAM) is subject to long day photoperiodic control in P. afra (L.) Jacq., a facultative CAM plant, was studied. Periodic measurements of 14CO2 uptake, stomatal resistance, and titratable acidity were made on plants exposed to long and short day photoperiods. Water-stressed P. afra had primarily nocturnal CO2 uptake, daytime stomatal closure, and a large diurnal acid fluctuation in either photoperiod. Mature leaf tissue from nonstressed plants under long days exhibited a moderate diurnal acid fluctuation and midday stomatal closure. Under short days, there was a reduced diurnal acid fluctuation in mature leaf tissue. Young leaf tissue taken from nonstressed plants did not utilize the CAM pathway under either photoperiod as indicated by daytime CO2 uptake, lack of diurnal acid fluctuation, and incomplete daytime stomatal closure. The induction of CAM in P. afra apparently is related to the water status of the plant and the age of the leaf tissue. The photosynthetic metabolism of mature leaves may be partly under the control of water stress and of photoperiod, where CAM is favored under long days.